Machines, including on- and off-highway haul and vocational trucks, wheel loaders, motor graders, and other types of heavy machinery generally include a multi-speed, bidirectional, mechanical transmission drivingly coupled to an engine by way of a torque converter assembly. The torque converter is a fluid coupling that multiplies and/or absorbs torque fluctuations transmitted between the engine and the traction devices of the machine by allowing slippage between an output shaft of the engine and an input shaft of the transmission. The torque converter assembly includes an impeller clutch to allow the operator a degree of freedom in determining the slippage between the output shaft of the engine and the input shaft of the torque converter (i.e., the impeller). The impeller clutch allows for high engine speeds while limiting the amount of torque that is transmitted into the torque converter, thus allowing some of the torque to be directed for other purposes.
As an operator depresses an impeller clutch pedal, the force keeping the impeller clutch engaged is proportionally reduced. As the force decreases, the clutch may begin to slip and less torque is transmitted from the engine to the torque converter. The amount of force applied to the clutch is selected such that, at high idle speed when the clutch pedal is not depressed, the clutch does not slip and substantially all of the torque generated by the engine is passed to the torque converter.
Although efficient at high idle speed, the magnitude of the force applied to the clutch when the pedal is not depressed may be sub-optimal at lower engine speeds. That is, the amount of torque generated by the engine may be somewhat proportional to the speed of the engine. As such, when the engine speed decreases, the torque produced by the engine likewise decreases. Thus, at a lower engine speed, less force is required to prevent the clutch from slipping because less torque is being produced by the engine and transmitted through the clutch. However, because the force applied to the clutch to maintain engagement corresponds with engine torque at high-idle speed, the force must be greatly reduced before any clutch slippage occurs. For this reason, modulation of the transmitted torque may be sluggish when the pedal is depressed at low engine speeds.
One system focused on improved impeller clutch control is disclosed in U.S. Pat. No. 5,613,581 (the '581 patent) issued to Fonkalsrud et al. on Mar. 25, 1997. The '581 patent discloses an electro-hydraulic control device for a drive train of a machine including an engine, a transmission, a torque converter, and an impeller clutch. A manually operated impeller clutch pedal produces an impeller clutch pedal signal in response to the position of the impeller clutch pedal. An impeller clutch electro-hydraulic valve produces fluid flow to the impeller clutch to controllable engage and disengage the impeller clutch. An impeller clutch pressure curve that is responsive to the impeller clutch pedal position is stored in memory. A rotary position switch selects a desired rimpull setting indicative of a desired reduction in rimpull and produces a desired rimpull signal. An electronic controller receives the desired rimpull signal and reconfigures the impeller clutch pressure curve. Thereafter, the electronic controller receives the impeller clutch pedal signal and controllably actuates the electro-hydraulic valve to produce a desired impeller clutch pressure defined by the impeller clutch pressure curve. This allows the operator to customize the reduced rimpull settings depending upon the machine operating conditions by way of improved pedal modulation.
Although the device of the '581 patent may improve control over the impeller clutch, it may still be sub-optimal. That is, having multiple impeller clutch pressure curves stored in memory may increase the cost and complexity of the system. And, because the system is manually activated, it may become burdensome on the operator. Further, the system still has some delays associated with operation of the machine at an engine speed less than high-idle, and the operational consistency experienced by an operator under varying engine speeds may be less than desired.
The disclosed control system is directed to overcoming one or more of the problems set forth above.